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Drawing a fine line

THE small print could soon become a lot smaller, thanks to nanotubes.
Researchers from Michigan State University and the University of Toronto have
designed a nanometre-scale “fountain pen” that they say will be able to
precisely deposit atoms on surfaces with a kind of pump. Although it is still a
design proposal and not a prototype, they say that all the technologies
necessary to make their instrument are currently in use.

If it can be made to work, the atomic pen could take over from fine line
lithography in microchip manufacturing, allowing the dimensions of written
features such as connecting wires and transistor contacts to move below today’s
micrometre-scale limits. It could also be used for atom-by-atom construction of
micromachines.

Developments in nanotechnology are hampered by a lack of tools for handling
parts on atomic and molecular scales. The best current method of atomic
manipulation, which uses a scanning tunnelling microscope (STM) to move single
atoms around a surface, is slow and laborious. But David Tomanek and Peter Kral
estimate that their atomic pump, which could be preloaded with atoms, might
deposit them on a surface at a rate of one every 15 microseconds.

The “ink cartridge” of the pump is made from a carbon nanotube—a sheet
of carbon atoms rolled into a tube—and can be filled with whatever atoms
are required. Two interfering laser beams, one with twice the frequency of the
other, control the flow of atoms. The lasers hit the tube and generate electrons
from the tube walls by a photoelectric effect. Interference between the laser
beams moves the electrons along.

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As they flow, the electrons exert a force on the trapped atoms, pumping them
along. By controlling the relative phase and power of the laser beams, an
operator can direct the speed and direction of the flow, spitting an atom out of
the tip of the tube whenever it reaches the right position.

Building the instrument will be an exacting test, the researchers say. Any
constrictions and defects in the nanotubes would have to be smoothed out to
avoid blocking the flow of atoms. Moreover, the geometry of the tip will be
important in ejecting atoms from the tube efficiently. The best design for this
is still unknown. Heating effects may also cause a problem, since very
high-powered lasers will be needed to generate a useful current.

In their paper, which is due to appear in a forthcoming edition of
Physical Review Letters, Tomanek and Kral suggest that atoms held between
nanotubes bundled together in a rope-like structure might clog less and provide
an easier escape from the tip. This mirrors Austrian work on moving individual
atoms down the outside of a wire (This Week, 13 March 1999, p 16). The heating
effects could be reduced by using short, 15-microsecond laser pulses: each pulse
would still be enough to move an atom the length of a nanotube, pumping it out
onto a substrate. Mauricio Terrones, a nanotube expert at the University of
Sussex, thinks the new work is clever and “will certainly motivate further
theoretical and experimental research in the field”.